Electronic counterparts have been developed for many different acoustic instruments. With the successful adoption of electronic keyboards and guitars, and the advent of a rich variety of synthetic devices implementing the MIDI (Musical Instrument Digital Interface) standard, electronic music instruments of many kinds are now in widespread use. An introduction to the techniques commonly used in the synthesis and transformation of sound and which form the basis of digital sound processing for music is presented in Digital Sound Processing for Music and Multimedia by Ross Kirk and Andy Hunt, Focal Press (1999), ISBN: 0240515064.
Conventional electronic percussion instruments typically employ a sensor as illustrated at 101 in FIG. 1 that is acoustically coupled to a drum or drum-like striking surface 103 for producing a timing signal that is processed by a triggerable digital direct sound module 105. The timing signal may also be created by attaching a pickup device called a “drum trigger” to an existing acoustic drum. A drum trigger or other sensor typically employs a pressure responsive piezoelectric transducer coupled to an amplifier and peak detector for producing trigger signals that indicate the timing at which a drum stick strikes the surface of the drum. Commercially available “drum kits,” such as the hi-hat electronic drum taught in Yamaha's U.S. Pat. No. 6,815,604, employ striking pads which simulate acoustic drumheads and other percussion instruments and employ striking surfaces that are struck with sticks (or striking rods). Striking intensities are detected by impact sensors such as piezoelectric transducers attached to the backs of the pads. The triggerable direct sound module 105 responds to each trigger signal by delivering an output signal to a conventional mixer and amplifier 107 connected to one or more loudspeakers 109. The output waveform produced by each striking event simulates, or is a recording of, the sound produced by the acoustic instrument being simulated. Triggerable direct sound modules are available from major manufactures such as Alesis, Roland, Yamaha and Kat.
In MIDI music systems, drum and other percussion sounds are simulated in response to a variety of trigger events, including keyboard events or drum pickups, which are converted into digital event signals conforming to the MIDI standard by a MIDI interface. A MIDI controllable sound module then produces digitized synthetic sound signals. A more description of an electronic percussion instrument of the type shown in FIG. 1 is presented in U.S. Pat. No. 5,293,000 issued to Alfonso Adinolfi on Mar. 8, 1994 entitled “Electronic Percussion System Simulating Play and Response of Acoustical Drum,” the disclosure of which is incorporated herein by reference.
The sound produced by both acoustic and synthetic instruments can be modified and enhanced to achieve special effects by a technique called “convolution.” Convolution, the integration of the product of two functions over a range of time offsets, and is a well known technique for processing sound. If an input sound signal is convolved with the impulse response of system (for example, the impulse response may represent the acoustic response of a particular orchestra hall), the signal produced by the convolution simulates the result that would occur if that sound signal had passed through a physical system with the same impulse response. Convolution has many known musical applications, including forms of spectral and rhythmic hybriding, reverberation and echo, spatial simulation and positioning, excitation/resonance modeling, and attack and time smearing.
The use of convolution in sound processing was presented as early as 1993 with the publication of the paper “Musical Sound Transformation by Convolution” by C. Roads, Proceedings of the International Computer Music Conference 1993, Waseda University, Tokyo. That paper contained an explanation of the theory and mathematics of convolution and included a survey of compositional applications of the technique as a tool for sound shaping and sound transformation. More recently, Roads described the uses of convolution in his book, The Computer Music Tutorial, MIT Press, 1996, pages 419-432 of which are devoted to convolution. Convolution has been used to create synthetic drum sounds.
Libraries of recordings of different acoustic drum sounds, recorded in an anechoic room, that can be triggered, for example, by a MIDI keyboard, are available. Many different versions of the same drum sounds are created by convolving the drum sounds with different recorded impulse responses exhibited by different rooms, or taken with different microphone locations in the room. The selection and combination of different drum sounds and different room characteristics as well as different microphone and instrument locations can be accomplished using available sound production software that includes the ability to convolve recorded sounds with the impulse response of different environments. See, for example, Larry Seyer Acoustic Drums for the GIGASTUDIO 3.0, Larry Seyer Productions, 2004.
All of the synthetic percussion instruments described above employ the same basic principle and suffer from a common disadvantage. Each sound or each simulated drum impact is initiated by a sensed or MIDI trigger event, indicating the timing of a drums stick impact or striking a key on a keyboard. When a striking surface is used, the output from the piezoelectric sensors is processed by peak detection to identify the trigger events. Thus, most of the information content of the signal from the impact sensor is largely discarded and only the event timing information is extracted to initiate the playback of a stored impact response.
As an example, U.S. Pat. No. 4,939,471 issued to Werrbach on Jul. 3, 1990 entitled “Impulse detection circuit” describes a triggering circuit for detecting drum beats within background noise and then triggering music synthesizers in response to the drum beat. As described in the Werrbach patent, differentiators, peak-rectifiers and filters are used to detect impulse like inputs over a wide dynamic range in a noisy background. The input signal is rectified and differentiated and then passed through a peak-rectifier and filter having a fast charging and a slow discharging time constant. The response of such triggering circuits is intentionally made highly-nonlinear in order to extract the only timing of substantial impacts on a drum pad surface, rejecting all other signals as being unwanted noise. As a result, the performer loses the ability to create and control many of the sounds and subtle effects that can be created with an acoustic instrument.